The Genius of Louis Pasteur

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Lawrence Livermore National Laboratory LLNL Announces Breakthrough In Free Electron Laser Work by Charles B. Stevens Details of a series of breakthroughs on the free electron laser were reported by scientists from Lawrence Livermore National Laboratory at the spring meeting of the American Physical Society in Washington, D.C., April 28- May2. In early 1985, the free electron laser, or FEL, was characterized by then presidential science advisor Dr. George Keyworth as making "revolutionary progress" in developing a laser for missile defense, leaping ahead 20 years. Keyworth told the Lasers '85 conference in December that the na tional laboratories had demonstrated accelerator technology for powering high-energy FELs, had demonstrated high-power FELs, and had demonstrated laser light propagation through the atmosphere. A single, ground-based FEL, operating with fighting mirrors popped up into space after detection of a missile attack, could have the firepower to destroy the entire Soviet ballistic missile inventory within a couple of minutes, Keyworth said. He suggested building a dozen such systems to leave no doubt in the mind of a potential aggressor The recent advances in the free electron laser used Lawrence Livermore National Laboratory's 5 million electron volt Experimental Test Accelerator as its source of fast-moving electrons. Shown here is the electron injector and pulse-forming network of the ETA. The large central cylinder houses the cathode structure and ferrite cores of the induction accelerators. that a surprise first strike could not work. Dr. Donald Prosnitz of Livermore presented the highlights of these breakthroughs at the APS meeting in a paper titled "Results of Free-Electron Laser Experiments—ED-3." The FEL is based on converting the energy of a high-energy electron beam into electromagnetic radiation. This is achieved by "wiggling" the electron beam with a magnetic field. The wiggler extracts energy from the electron beam and transforms it into electromagnetic radiation. The FEL can be tuned by changing the dimension and intensity of the magnetic wiggler. It also leads to lasing efficiencies orders of magnitude greater than those based on more conventional atomic lasers. The FEL Advances "We have improved the efficiency of free electron laser amplification of high power microwaves from 5 percent to 40 percent by replacing the straight wiggler with a tapered one," Prosnitz reported. "In experiments over the last year, we have amplified 15 nanosecond (1 nanosecond is one billionth of a second), 50,000 watt peak power microwave pulses produced by a magnetron 20,000 times (43 decibels) to 1 billion watts peak power. This is clearly the most powerful source of coherent electromagnetic radiation atthiswavelength with efficiencies nearly as good as the most efficient low-power sou rces (klystrons). The microwave signal we amplified had a wavelength of 8.67 millimeters (34.6 billion cycles per second or gigahertz)." The recent experiments were conducted at the Electron Laser Facility (ELF) at Lawrence Livermore National Laboratory. The ELF uses the 5 million electron volt (5 MeV) Experimental Test Accelerator as its source of fast-mov- 48 September-October 1986 FUSION Beam Technology Report
ing electrons. A free-electron laser amplifier is a device that can transfer energy from a beam of electrons traveling at nearly the speed of light to a beam of electromagnetic radiation, such as microwaves or visible light. The energy transfer increases the intensity of the electromagnetic radiation. The advantage of the free-electron laser amplifier is its efficiency in converting electrical energy into radiation and its ability to be tuned over a wide range of wavelengths. The Livermore approach has the additional benefit of being able toproducevery high power pulses of radiation. High power microwaves from a freeelectron laser might be able to drastically reduce the length and power requirements of linear charged particle beam accelerators by using a "twobeam" design proposed in 1982 by Dr. Andrew M. Sesslerat Lawrence Berkeley Laboratory. The powerful microwave source could also help heat fusion fuel in some magnetic fusion reactor designs. Another application would be microwave power for spaceships and aircraft. FEL Wiggler A 10-foot-long array of powerful, 5,000-gauss magnets with alternating north and south poles was used in the ELF wiggler. The high-energy electrons in the accelerator beam oscillate back and forth when they pass through the wiggler. This regular change of direction causes the electrons to give off electromagnetic radiation at wavelengths characteristic of the distance over which the oscillation takes place. If the electron beam is accompanied by a beam of electromagnetic radiation (which in ELF consisted of microwaves) with a wavelength that exactly matches the electron's oscillation, the electron gives up some of its energy to the radiation beam. The energy transfer both makes the radiation more intense and slows the speeding electrons slightly. To compensate for the electron's energy loss as it passes through the wiggler, the wiggler is magnetically tapered. This is done by reducing the magnetic field strength along the wiggler's length to exactly match the reduced energy of the electron beam. In this way the electron oscillation is kept in tune with the wavelength of the radiation output throughout the length of the wiggler. The combination of the properly tapered magnetic field and added length is what significantly increased the power and efficiency of the free-electron laser amplifier over results reported in 1984 using a simple, straight wiggler. In these experiments, 80 megawatts peak power was produced—a 2,500-fold amplification at an efficiency of 5 percent. Alternative Applications In addition to providing the basis for missile defense, the existing microwave FEL could be used to drastically reduce the length and power requirements of linear charged particle accelerators, and to heat and confine thermonuclear fusion plasmas. The accelerator application was proposed in 1982 by Sessler, whose twobeam particle accelerator concept would use two parallel beams of electrons. The first would be an intense, but low-energy beam for free-electron lasing. The second would consist of compact bunches of very high energy electrons. The microwaves from the FEL would be fed into the high energy beam every foot or so. This is 10 times better than the 10foot transfer regions achieved with existing klystron-driven accelerators. Thus, the result would be a high-energy linear accelerator that is not only more efficient than present designs, but also is either 10 times shorter or 10 times more energetic than is now possible. Microwaves have long been used to confine and heat hydrogen fuel at the high temperatures required for thermonuclear fusion. Recently, experiments on the magnetic tokamak have shown that microwaves can be used to maintain electric currents in thermonuclear plasmas. This has led to prospects for "steady-state"—as opposed to pulsed mode of operation—fusion reactor designs. The current Livermore FEL breakthroughs will revolutionize every aspect of magnetic fusion research. There are even designs, chiefly developed by Soviet scientists, for microwave-driv- Continued on page 63 HOW THE FREE ELECTRON LASER WORKS The free electron laser converts the energy of a high-energy electron beam into electromagnetic radiation. A beam of high-energy electrons enters the free electron laser together with a pulsed laser field. Magnets arrayed in alternating gradients (north and south) on the device cause the electrons to "wiggle, "releasing energy to the beam. At a certain distance along the device, the kinetic energy of the electrons is transferred to the input laser beam, thus amplifying it. The FEL can be tuned by changing the dimension and intensity of the magnetic wiggler. Recent advances have improved the efficiency of FEL amplification of high power microwaves from 5 percent to 40 percent by replacing the straight wiggler with a tapered one. Beam Technology Report FUSION September-October 1986 49
Page 1 and 2: The Genius of Louis Pasteur His Dis
Page 3 and 4: FUSION SCIENCE • TECHNOLOGY • E
Page 5 and 6: Letters Put Fusion in Every School!
Page 7 and 8: strong, stable, homogenous magnetic
Page 9 and 10: SDI OPPONENTS PRESENT PETITION TO U
Page 11 and 12: just a containment structure around
Page 13 and 14: Fusion Report Fusion at the 'Breako
Page 15 and 16: H-mode has appeared only in tokamak
Page 17 and 18: university institute in a governmen
Page 19 and 20: Conference Report Warren Hamerman T
Page 21 and 22: Louis Pasteur After a photograph of
Page 23 and 24: ing classical geometric "projection
Page 25 and 26: action of the twisted staircase or
Page 27 and 28: light. The extent of rotational act
Page 29 and 30: could be as similar as Mitscherlich
Page 31 and 32: Figure 8 HUYGENS'S PRINCIPLE AND CI
Page 33 and 34: thetartrate/paratartrate problem th
Page 35 and 36: esponsbility to maintain leadership
Page 37 and 38: Illustration by Robert McCall, cour
Page 39 and 40: Illustration by Robert McCall. cour
Page 41 and 42: Chairman Talks About America in Spa
Page 43 and 44: and provides a fundamental insight
Page 45 and 46: Courtesy of George Washington Unive
Page 47 and 48: Beam Technology Report Teller Confi
Page 49: For directed energy weapons, beam b
Page 53 and 54: Figure 1 THE NUCLEAR FISSION PROCES
Page 55 and 56: • the fuel rods of zirconium allo
Page 57 and 58: tion. This glow makes it harder to
Page 59 and 60: y liquid nitrogen to keep the liqui
Page 61 and 62: On Volcanoes, Sunspots, and the Jup
Page 63 and 64: Dr. Joseph M. Hendrie Continued fro
Page 65 and 66: Free Electron Laser Continued from
Page 67 and 68: Green Terrorists at War Continued B

The Genius of Louis Pasteur

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